Could just be that it lost some potential by itself by running faster anyway.
I know my old AXP 2500+ won't hit 3200+ speeds anymore (locks up in Windows) even though it used to run at those speeds all the time. I don't see GPU's being any different.
Sometimes the reason those pipes were locked in the first place is that they were damaged. It's dissapointing that you can no longer OC, but count youself blessed that the card runs at all with those 4 pipes unlocked.
For fun, you may try "locking" those pipes up again, and see if the card will run faster.
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